Opticks: Or, A Treatise of the Reflections, Refractions, Inflections, and Colours of Light

Overview
Published in 1704, Isaac Newton's Opticks presents a sweeping experimental investigation into the behavior of light. Written in clear English rather than scholarly Latin, it advances a program grounded in carefully staged experiments, quantitative measurement, and logical deductions. The treatise recasts color as an intrinsic property of light, not a modification imparted by media, and builds a framework encompassing reflection, refraction, dispersion, thin-film phenomena, and the bending of light at edges. A series of Queries appended to later editions extends the scope to bold hypotheses about ether, forces in matter, heat, chemical action, and the relation of light to particles.

Decomposition of White Light
Newton’s prism experiments show that white light is heterogeneous. A beam refracted by a prism spreads into a spectrum because it contains rays of different refrangibility; red rays bend least, violet most. By isolating a single colored ray and passing it through a second prism, he demonstrates its color is fixed and not further decomposed. The celebrated experimentum crucis, using two prisms in opposed orientations, proves that colors are not created by glass but separated by refraction according to intrinsic properties. Recombining the spectrum recovers white light, establishing color mixing as additive. This refrangible-irrefrangible structure underpins his account of natural phenomena such as rainbows and atmospheric colors.

Refraction, Dispersion, and Vision
Opticks develops precise laws of refraction and reflection and ties refraction to dispersion: the refractive index of a medium varies with color. From this Newton infers the inevitability of chromatic aberration in refracting lenses, where different colors focus at different distances. He critiques schemes for correcting it by combining glasses of varied density and argues that only reflecting telescopes can avoid the defect, a position aligned with his earlier invention of the reflecting telescope. The analysis links the anatomy of the eye, accommodation, and the sensation of color to the physical composition of light.

Reflection, Thin Films, and Fits
Through experiments with polished metals and glass, Newton confirms the equality of angles of incidence and reflection, but he also discovers periodic color patterns in thin films, soap bubbles, wedge-shaped glasses, and the contact of a lens on a flat plate. These “Newton’s rings” vary with thickness and illumination and repeat at regular intervals. To explain the alternations of brightness and darkness upon reflection and transmission, he introduces “fits of easy reflection and refraction,” periodic dispositions of light that render it alternately more apt to reflect or to refract. Though framed corpuscularly, the observations anticipate later wave accounts of interference.

Inflection and the Bending of Light
In the study of inflection, Newton measures the slight lateral deviations and fringes formed when light passes near edges or through narrow apertures. He treats these border phenomena as alterations in the paths of rays near bodies and extends the notion of fits to account for the spacing and colors of the fringes. The precise patterns he catalogues become a testbed for later diffraction theory.

Queries and Natural Philosophy
The Queries venture beyond established results. Newton asks whether a subtle elastic medium pervades space, whether light excites vibrations in such an ether, and whether short-range attractive and repulsive forces govern cohesion, capillarity, and chemical affinity. He speculates on heat as motion, on combustion and fermentation as processes mediated by active particles, and famously on the possible convertibility of light and matter. These conjectures, offered as questions, sketch a program for integrating optics, chemistry, and mechanics.

Legacy
Opticks reshapes optics by making experiment the arbiter and by redefining color as a property of light. Its methods, results, and provocative Queries dominated eighteenth-century physical science, catalyzed debates over particle versus wave theories, and supplied durable tools and phenomena, from dispersion and spectral analysis to Newton’s rings, that continue to structure optical inquiry.